This invention relates generally to nuclear reactors and more particularly to a bearing and sealing system for nuclear reactors utilizing rotating members above the nuclear reactor vessel.
The nuclear reactor produces heat by fissioning of nuclear materials fabricated into fuel elements and assembled within a nuclear core situated in a pressure vessel. In commercial nuclear reactors, the heat produced thereby is used to generate electricity. Such nuclear reactors typically comprise one or more primary flow and heat transfer systems, and a corresponding number of secondary flow and heat transfer systems. These secondary heat transfer systems are coupled to conventional steam turbines and electrical generators. A typical energy conversion process for a commercial nuclear reactor, therefore, involves transfer of heat from a nuclear core to the primary coolant flow system, to a secondary coolant flow system, and finally into steam from which electricity is generated.
At the top of the pressure vessel, it is customary to provide a head to seal the nuclear reactor. A plurality of penetrations pass through this head into the reactor vessel below. These penetrations, typical of which are control rod assemblies and transfer machines, perform functions within the nuclear pressure vessel.
To assure complete access to all areas of the pressure vessel, particularly for liquid metal cooled, breeder reactors these penetrations are generally mounted on rotatable members such as rotating plugs. These rotating plugs are generally cylindrical, and are of decreasing size, the innermost plug being the smallest and the outermost plug being the largest. For the most efficient operation, each plug is radially offset from each other plug, and each plug is supported by the next largest plug.
During operation, each plug is rotated independently of other plugs. Additionally, each plug supports penetrations which may rotate independently of the plug.
It is customary for each rotating plug member to be connected to its supporting member by means of a load bearing. This type of connection permits rotation while maintaining support. To permit this rotation, the rotating members cannot otherwise be physically joined together. Because this connection occurs above the nuclear reactor vessel, sealing means must be utilized to prevent the escape of any gases present in the pressure vessel through the annulus between the two members. Such gases may comprise reactor vessel cover gases which are positioned above the liquid metal level in the pressure vessel of a liquid metal cooled reactor and/or may include fission gases released from the nuclear level during reactor operation. These sealing means are seals attached to one of the members, and kept in sliding contact with the other member.
A problem arises in the course of nuclear reactor operations. the seals and the bearings are in friction contact with the members during rotation. These friction forces will eventually cause both the seals and the bearings to lose their expected effectiveness.
Because the location of the seals and bearings are above the reactor vessel, they cannot be easily lubricated to minimize friction loses. Any excess lubrication present would drain down the annulus between the two members and into the reactor cavity. There, this lubrication may cause undesirable reactions with the reactor components.
Another problem occurs when changing the bearings or the seals. To prevent an undesirable leakage of radioactive gases into the atmosphere, the pressure vessel is desirably kept in an airtight enclosure. The usual means of accomplishing this during reactor operation is by means of the seals. However, some other mechanism must be utilized when the seals themselves have to be changed. Prior practice in the field was to place an airtight enclosure over the entire head area. Then, the seals and the bearings were changed by machinery controlled external to the enclosed head. This method requires the use of complex machinery and control mechanisms, and results in a long stoppage of operation of the nuclear reactor.
Another method utilized is to pour a liquid metal into a trough provided for it in the annulus, and allow it to freeze, sealing the annulus. This method is not entirely satisfactory, as the metal could overflow from its trough and drain into the reactor pressure vessel.